Recently, there has been a growing interest in alternative energy sources as depletion of existing energy resources such as petroleum or coal is expected. As one alternative energy source, in particular, fuel cells are receiving much attention due to high efficiency, no discharge of pollutants such as NOx, SOx, and the like, and abundance of fuels used.
Fuel cells are power generation systems that convert chemical reaction energy of a fuel and an oxidizing agent into electrical energy. Hydrogen, methanol, or a hydrocarbon such as butane or the like is used as the fuel, and oxygen is used as the oxidizing agent.
In polymer electrolyte fuel cells, the most basic unit that generates electricity is a membrane electrolyte assembly (MEA), which includes a polymer electrolyte membrane and an anode and cathode disposed on opposite surfaces of the polymer electrolyte membrane. Referring to FIG. 1 and Reaction Scheme 1 (reaction scheme of a fuel cell using hydrogen as a fuel) that illustrate a principle of electricity generation of fuel cells, oxidation of the fuel occurs at an anode to generate hydrogen ions and electrons, the hydrogen ions move to a cathode via a polymer electrolyte membrane, and oxygen (oxidizing agent), the hydrogen ions transferred via the polymer electrolyte membrane, and the electrons react at the cathode to produce water. Transfer of electrons to an external circuit occurs by such reaction.Anode: H2→2H++2e−Cathode: ½O2+2H++2e−→H2OOverall reaction scheme: H2+½O2→H2O  [Reaction Scheme 1]
In this reaction, the polymer electrolyte membrane undergoes changes in thickness and volume of 15 to 30% according to temperature and a degree of hydration and, in particular, undergoes changes in volume of up to 200% or greater by 3 to 50 wt % of methanol as a fuel. Accordingly, the polymer electrolyte membrane repeatedly undergoes swelling and contraction according to operating conditions of a fuel cell and, due to such changes in volume, polymer chains of the polymer electrolyte membrane are disentangled and thus the polymer electrolyte membrane has reduced mechanical strength and micropores or cracks are formed therein. Through such micropores or cracks, hydrogen or methanol crossover occurs, which is a main cause of reduction in durability of fuel cells.
For these reasons, fluorine-based cation exchange resins with high conductivity and excellent mechanical/physical properties and chemical resistance are mainly used as the polymer electrolyte membrane. In particular, a perfluorosulfonic acid resin membrane prepared using perfluorosulfonic acid resin (Model name: Nafion) is mainly used.
The perfluorosulfonic acid resin has a tensile strength of 26 to 34 MPa at room temperature in a wet state and a tensile strength of 32 to 43 MPa at 50% relative humidity. The tensile strength of the perfluorosulfonic acid resin has no problem under general operating conditions of a fuel cell, but the perfluorosulfonic acid resin membrane requires superior mechanical properties because fuel cells used in vehicles and the like require smooth operation under more stringent conditions.
In general, to enhance durability of a polymer electrolyte membrane for fuel cells, a method of enhancing strength of an electrolyte membrane resin itself or a method of filling a porous substrate with an electrolyte membrane resin is used. However, when the former method is used, in general, ion exchange capability is reduced and, when the latter method is used, durability improvement effects are obtained while there are many difficulties in manufacturing processes and raw material costs increase. As another method, there is a method of mixing an electrolyte membrane resin and a material for enhancing durability, but the mixing process is not easy and, above of all, distinct effects are not obtained.
Therefore, those working in the related field had put much effort into addressing these problems and thus completed the present invention.